In order to study the influence of flow instability under off-design operating conditions, based on the SST k-ω turbulence model, the generation and evolution mechanism of unsteady flow and the pressure pulsation characteristics of the vaneless space (VS) were studied under four offdesign operating conditions with three guide vane openings (GVO). The results showed that under the exceptionally small GVO (G11), the disturbance from the backflow of the tailwater pipe in the outlet area of the impeller was significant. With the increase of the GVO, the influence of this disturbance was gradually decreased. The phenomenon of blade vortex and flow separation also showed a decreasing trend with the increase of the GVO. The high turbulent kinetic energy (TKE) zone was mainly distributed in the upper reaches of the runner blade channel and the VS, the turbulent kinetic energy distribution zone was large under the turbine under low flow and runaway conditions, and the TKE zone was mainly distributed in the VS and the runner blade outlet under braking conditions. At small blade opening angles, vortices were generated under all four operating conditions, and the vortex core distribution was dense. As the blade opening angle was increased, the distribution of vortex cores was significantly weakened. Under the high flow condition, vortices mainly appeared in the form of central vortex bands, and as the random group flow rate was decreased, the distribution of vortex cores was gradually strengthened on the pipe wall side. Under abnormally small GVO, the low-frequency component (LFC) and high frequency-low amplitude component (HF-LAC) of the pressure pulsation in the VS were extremely minimal. Under the offdesign condition with large opening, the BPF component and LFC jointly became the dominant frequency of the pressure pulsation in the VS. Under abnormally small GVO, the flow structures in each flow domain had significant specificity compared with that under the large GVO, significantly impacting the pressure pulsation in the VS.